1. Field
The invention relates to the field of transmission of signals and more particularly to the transmission of signals over cables.
2. Background Information
Digital devices may be coupled over a cable in order to exchange signals. The cable may be a single copper wire conductor or a pair of copper wire conductors twisted together in what is known as a xe2x80x9ctwisted pairxe2x80x9d configuration (among numerous possibilities). The Institute of Electrical and Electronics Engineers (IEEE) and other standards bodies have defined cable configurations for which signaling characteristics are well defined. One such standard involves twisted pair cable employed to carry Ethernet data traffic and is called International Standards Organization (ISO) standard IEC SPEC No. 11801:1995 (or CAT-5 for short). CAT-5 provides for signal transmission at up to 125 million baud (125 Mbaud) over copper twisted pair cable at lengths up to 100 meters.
Cables may have signaling characteristics which distort signals transmitted over the cable as transmission length increases. For example, at cable lengths approaching 100 meters, binary signals on a CAT-5 cable may degenerate from an approximately rectangular shape into a shape with less well defined edges. Furthermore, phase shifts in the digital signal may occur as transmission distance increases.
These changes in signal properties may be especially prominent as the frequency content of the signal increases. A binary digital signal may be divided into its frequency components using a Fourier Transform. As transmission length increases a more noticeable attenuation of amplitude and shift of phase may be observed for the higher frequency components (for example, frequency components over 1 megahertz) of the signal than for the lower frequency components.
To compensate for the non-uniform frequency response of cables, equalizers may be employed on receiver devices along the cable. The equalizers may comprise passive filter components (such as resistors and capacitors) to allow higher frequency components of the signal to pass while attenuating lower frequency components of the signal to adjust for the attenuation and phase shift to the high frequency components caused by the cable. However, the attenuation to apply may vary according to the distance along the cable of the receiver from the transmitter. Receivers closer to the transmitter may employ little or no attenuation while receivers further distant may employ substantial attenuation to equalize the effects on higher frequencies imposed by the cable.
It is typically not known at manufacturing time at what distance from a transmitting device a receiving device may be located along the cable. It may therefore be necessary for the receiving device to employ an adaptive equalizer which may detect the amount of attenuation imposed by the cable on higher signal frequencies and adapt the filter characteristics to compensate accordingly. Adaptive equalizers may be more complex and expensive than other equalization solutions, and may require more passive components.
Signals transmitted on a cable are filtered at a transmitter to equalize for non-uniform frequency response of the cable at a maximum cable length. The signals are clipped at a receiver when the filtering results in overshoot of a signal level by the signals.